8 
DR R. C. DAVIE ON 
leaf-trace {ibid., fig. 9) there is a prominent abaxial tongue. P. murorum, Hk., with 
the shortest leaf of the four, has little or no abaxial development of xylem in the 
leaf- trace {ibid., fig. 10), though the rather broad pinnae are probably the cause of 
the greater adaxial development than that in the leaf-trace of P. Mayoris. 
The greater amount of xylem present in the petioles of P. crassifolium, P. 
angustifolium, and P. murorum, compared with that in P. Mayoris, is apparently 
related to the habitats of the Fern. The first three are xerophytes, growing on 
high, sunny, dry places {loc. cit., p. 14) ; P. Mayoris is a mesophyte. Thus both in 
relation to habitat and to the size of the leaf these Colombian Ferns show close 
structural parallels to the series of Brazilian Ferns described above. 
The Leaf-Trace of Polypodium vulgare, Linn. 
A more limited field than that afforded by the South American Polypodiums, 
but one which provides ample scope for testing the accuracy of the conclusions 
already stated, is given by P. vulgare, Linn. Plants of this species were collected 
in various situations and at different altitudes in several parts of Scotland. Some 
a b c d e f 
Text-fig. 2. 
of these plants had leaves only two inches long ; in others the leaves were as 
long as eighteen inches. Some leaves were leathery, others much less so. Plants 
were collected from wall-tops, rocks, hedge-rows, stream-sides, tree-trunks, the 
depths of woods, the open hillside, the sea-coast. The types of leaf-trace found in 
these plants are diagrammatically represented in text-fig. 2. In short leaves the 
tracheide-group forms a narrow plate stretching from one side of the petiole to the 
other (text-fig. 2 a ) ; in longer leaves it becomes an isosceles triangle (in section) 
with the apex on the abaxial side of the petiole (text-fig. 2b) ; the triangular outline 
appears also in some of the longest leaves (text-fig. 2 d,f). But in some others two 
separate strands low down in the petiole unite below the first pinnae to make an 
inverted V (text-fig. 2c) ; or two groups remain separate from each other until far 
up the rachis — one a large wide plate towards the adaxial side, the other a small 
round group towards the abaxial face (text-fig. 2e). In passing from the shortest 
leaves to the longest, through a series of intermediate forms, there is a gradual 
appearance of an abaxially-directed tongue in the leaf-trace {cf. text-fig. 2b, d,f). 
Such a tongue we have already seen becoming more prominent the longer the 
leaf, and finally being replaced by a system of separate strands along the abaxial 
curve of the petiole, in the species of Polypodium from Brazil and Colombia. 
Within the narrow limits of one species- of Poly podium — & species very variable, 
however, in its habitats — there is the same variation in the form of the leaf- 
